Thanks for the specs, but it's only partial information.
Based on that partial information, I have some recommendations:
If you installed the KLite Codec Pack, you need to do a total and complete system reinstall from the ground up, ESPECIALLY the operating system. This is NO JOKE: this is dead-serious advice from Adobe's Todd Kopriva.
Since we don't know your AE version, nor the effects applied to the footage, you should transcode the video footage to a Quicktime Movie in the PNG codec to eliminate the possibility of long-gop codecs causing problems.
Turn off multiprocessing. 8 gigs may seem like a lot, but it really isn't in AE terms.